40 years of Dogger aquifer management in Ile-de-France, Paris Basin, France

International audienceGeothermal energy has been supplying heat to district networks in the Paris Basin for more than 40 years. The most serious difficulties have been corrosion and scaling related problems that occurred in many geothermal loops in the mid-1980s. The main target of all exploration and exploitation projects has been the Dogger aquifer. Most of the operating facilities use the “doublet” technology which consists of a closed loop with one production well and one injection well. Injection of the cooled brines leads to the progressive exhaustion of the resource at the local doublet scale. Consequently, most of the research effort has been focused on quantifying the temporal evolution of the cooling, and to forecast the lifetimes of doublets and the occurrence of the “thermal breakthrough”. At the turn of the 21st century, there was a revival of geothermal energy development in France and new projects are presently being considered. The 40 years of experience in geothermal exploitation of the Paris basin constitutes a firm basis upon which to devise a sustainable regional management approach for the geothermal resource. Several governmental policies seek to promote further geothermal development of the Dogger aquifer with structures in place to facilitate technical studies

Clogging of shallow geothermal doublets: hydrologic, geochemical and microbiological observations in a pilot site

International audienceWell and surface equipments (exchangers, filters) clogging is a major issue that affects many geothermal doublets in shallow aquifers (open-loop groundwater heat pump). In the case of wells, injection wells in particular, clogging of screens, porosity of gravel pack filters, and more widely the pores of the aquifer lead to severe losses in hydraulic performance in a long run. These clogging processes involve complex interactions between abiotic chemical processes (precipitation of carbonate minerals; iron and manganese oxides), physical processes (mobilisation and precipitation of particles) and biological processes (mineral precipitation linked to bacterial metabolism or biofouling). Each of these processes is well known and documented individually. However, prediction of clogging phenomena using a global approach that could integrate parameters related to operating conditions (temperature, pressure, flow rate...) as well as water chemical, physical, and biological characteristics is a scientific challenge. Except for carbonates species, the small temperature variation (<15°C) induced by geothermal doublet operation seems to have a limited effect on the concentration of the major chemical components of groundwater but does impact redox equilibria. The main impact on hydrochemical characteristics and equilibrium states of the groundwater induced by geothermal doublets operating at injection temperatures below 30°C is related to mixing of waters with different chemical facies (Bonte et al, 2013; Possemiers et al, 2016). From a biological point of view, the microbial communities responsible for clogging are highly sensitive to temperature variations, redox variations, and operating conditions (Vetter et al, 2012). Redox equilibria and their influence on biochemical reactivity are very sensitive to very small changes in physico-chemical parameters. Such variations might occur on surface or in-depth installations due to the impact of pumping and/or injection. The impact of mixing of waters with different chemical compositions on biochemical reactions in a context of heterogeneous geological environments is a research question debated over recent years. We present the initial results of a research project carried out jointly by Antea Group and the Geosciences Rennes laboratory, launched in April 2015 aiming at studying the origin of clogging problems and analyzing the effects of preventive and curative treatments. In this framework, field investigations of geothermal doublets are performed to evaluate and identify the possible origins of clogging phenomena. For selected doublets, a real-time monitoring of operating conditions and water physico-chemical characteristics is performed. In addition, chemical and bacteriological analyzes are carried out on deposits and water samples. The first challenge is linked to the selection of the most relevant geochemical and hydrogeological tools to characterise clogging of geothermal doublets under operation. The second issue is related to the characterisation and simulation of biogeochemical reactions induced by the operation of a geothermal doublet. In this context, the evaluation of reaction mechanisms and controlling parameters is critical to assess the clogging risk of an installation and to select the appropriate preventive or curative solutions

Impact of mixing chemically heterogeneous groundwaters on the sustainability of an open-loop groundwater heat pump

International audienceGeothermal systems using shallow aquifers are commonly used for heating and cooling. The sustainability of these systems can be severely impacted by the occurrence of clogging process. The geothermal loop operation (including pumping of groundwater, filtering and heat extraction through exchangers and cooled water injection) can lead to an unexpected biogeochemical reactivity and scaling formation that can ultimately lead to the shutdown of the geothermal doublet. Here, we report the results of investigations carried out on a shallow geothermal doublet (< 40 m depth) affected by rapid clogging processes linked to iron and manganese oxidation. Using a reactive transport model, we determine the parameters controlling clogging. To characterize the biogeochemical processes induced by the operation of the production well, we combined hydrodynamic measurements by flowmeter and in-situ chemical depth profiles. We thus investigated the chemical heterogeneity into the pumping well as a function of the operating conditions (static or dynamic). Hydrochemical data collected at the pumping well showed that groundwater was chemically heterogeneous long the 11 meters well screen. While the aquifer was dominantly oxic, a localized inflow of anoxic water was detected and evaluated to produce about 40% of the total flow . The mixture of chemically heterogeneous water induced by pumping lead to the oxidation of reductive species and thus to the formation of biogenic precipitates responsible for clogging. The impact of pumping waters of different redox potential and chemical characteristics was quantified by numerical modeling using PHREEQC. These results shows that natural chemical heterogeneity can occur at a small scale in heterogeneous aquifers and highlight the importance of their characterization during the production well testing and the geothermal loop operation in order to take preventive measures to avoid clogging